<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>flex</title>
  <style>
    #box{
      width: 100%;
      height: 700px;
      display: flex;
      margin: 10px;
    }
    #left,
    #right{
      width: 400px;
      height: 700px;
      margin: 10px;
      background-color: darkviolet;
     
    }
    #center{
      flex: 1; 
      height: 700px;
      margin: 10px;
      background-color: blanchedalmond;
    }
  </style>
</head>
<body>
  <div id="box">
    <div id="left"></div>
    <div id="center"></div>
    <div id="right"></div>

    
  </div>
</body>
</html>